1. Field of the Invention
The present invention relates generally to a method of manufacturing a phosphor screen of a cathode ray tube. More particularly, this invention relates to a method of manufacturing an intermediate layer for a metal back layer.
2. Description of the Prior Art
As a method of manufacturing a phosphor screen of a color cathode ray tube, there is known a so-called PVA (polyvinyl alcohol) slurry method.
In order to understand the present invention more clearly, let us first explain this PVA slurry method with reference to process diagrams forming FIGS. 1A to 1F.
As shown in FIG. 1, a face plate 21 is prepared, and a light absorption layer, for example, a carbon stripe layer 20 is formed on the face plate 21. Ammonium bichromate is added to a polyvinyl alcohol solution and a phosphor is mixed into the resultant solution to form a so-called phosphor slurry 22. The phosphor slurry 22 is coated on the inner surface of the face plate 21, dried and is then exposed to light by using a color selection electrode (for example, aperture grille) as an optical mask (see FIG. 1A). After the exposure, the color selection electrode is removed and the product is developed by water, whereby the portion irradiated with light is left thereon to form a phosphor layer, for example, a phosphor stripe 23. The similar processes are repeatedly carried out to form a green phosphor stripe 23G, a blue phosphor stripe 23B and a red phosphor stripe 23R, sequentially (see FIG. 1B).
The product is then dried and is uniformly coated with an aqueous solution 24 containing an acrylic resin (for example, "PRIMAL", product name) as shown in FIG. 1C. Then, the product is again dried and an acrylic resin film, or an intermediate film 25 is formed on the phosphor stripe 23 as shown in FIG. 1D. Thereafter, a metal back layer 26 is formed on the intermediate layer 25 by an aluminum vapor deposition process (see FIG. 1E), and the whole of the product is baked to remove the intermediate layer 25 beneath the metal back layer 26. Thus, the process for manufacturing a phosphor screen is ended as shown in FIG. 1F.
The metal back layer 26 has a charge-up effect for the lowering of the surface potential of the phosphor screen by the bombardment of electrons from an electron gun or such an electrical effect that the surface potential of the phosphor screen is maintained to be an anode potential. Also, the metal back layer 26 has such an optical effect that a reflection coefficient is increased by using the aluminum thin film forming the metal back layer 26 as a mirror surface. Further, the metal back layer 26 has such an effect that can prevent ion spot from being produced when negative ion within the cathode ray tube strikes the phosphor screen, or the metal back layer 26 can prevent brightness of a phosphor screen from being deteriorated or the metal back layer 26 can increase the brightness of the phosphor screen. If the metal back layer 26 is smooth, then the above-mentioned effects become more remarkable. Therefore, it is proposed in the art that the metal back layer 26 is made smooth by forming the intermediate layer 25 on the phosphor stripe 23 prior to the aluminum vapor deposition process.
The prior-art method of manufacturing a phosphor screen of a cathode ray tube will be described more fully with reference to FIGS. 2A and 2B. As shown in FIG. 2A, when the acrylic resin solution 24 is coated on the phosphor stripe 23, the solution 24 is permeated into the phosphor materials 23a. If the product is dried under this condition, then the intermediate layer 25 is formed on the surface of the phosphor stripe 23 so as to fill in its concavities and convexities.
The intermediate layer 25 is, however, formed on the surface of the phosphor stripe 23 in accordance with the large concavities and convexities formed on the surface of the phosphor stripe 23. Consequently, the intermediate layer 25 itself is not formed smooth so that the metal back layer 26 formed on the intermediate layer 25 is not formed smooth, as shown in FIG. 2B. As a result, the effects inherent in the metal back layer 26 can not be demonstrated sufficiently.
In order to make the intermediate layer 25 more smooth, the film thickness of the intermediate layer 25 is increased by increasing a concentration of the acrylic resin in the solution, thereby filling in the concavities and convexities on the surface of the phosphor stripe 23. In this case, however, upon the baking-process, a relatively large amount of the intermediate layer 25 is sputtered and the metal back layer 26 formed on the intermediate layer 25 is raised, which provides a problem of a so-called expanded aluminum film (or floated aluminum film). This causes the brightness of the cathode ray tube to be deteriorated. For this reason, the film thickness of the intermediate layer 25 is limited, or the concentration of the acrylic resin in the solution is limited (the limit is generally about 30%). Thus, the intermediate layer having satisfactory smoothness can not be obtained.
In order to make the intermediate layer smooth, other methods are proposed. One of such previously-proposed methods is to form the intermediate layer by the use of acryl lacquer. This method, however, needs some special apparatus for spraying acryl lacquer on the phosphor material. Also, the acryl lacquer is an organic solvent and has to be treated with great care.